An ignition plug is attached to an internal combustion engine (engine) etc. and used to ignite, for example, an air-fuel mixture in a combustion chamber. Generally, an ignition plug includes an insulator having an axial hole extending in an axial direction, a center electrode inserted into a forward end portion of the axial hole, a metallic shell disposed externally of the insulator, and a ground electrode fixed to a forward end portion of the metallic shell. The insulator is fixed to the metallic shell in a state in which a step portion provided on the outer circumference of the insulator is engaged with an inner circumferential portion of the metallic shell directly or through a metal-made sheet packing. A spark discharge gap is formed between a distal end portion of the ground electrode and a forward end portion of the center electrode. A high voltage is applied across the spark discharge gap to generate spark discharge, whereby an air-fuel mixture, for example, is ignited.
In engines proposed in recent years, to improve fuel economy and to cope with environmental regulations, the degree of supercharging and the dree of compression, for example, are increased. In these engines, since the pressure inside each combustion chamber during operation is relatively high, the voltage necessary to generate spark discharge (discharge voltage) is also high. When the discharge voltage is high, spark discharge passing through the insulator (penetration discharge) may occur in a leg portion of the insulator which is located forward of the step portion and whose wall thickness is relatively small, and this may hinder normal spark discharge (may cause a misfire). Particularly, in recent years, to achieve a reduction in ignition plug size, the insulator is further reduced in wall thickness. The possibility of the occurrence of penetration discharge is particularly high in such a thin-walled insulator.
A possible measure for suppressing the occurrence of penetration discharge is increasing the denseness of the insulator, i.e., reducing the porosity of the insulator, to thereby increase the dielectric strength of the insulator. In one previously proposed technique (see, for example, Japanese Patent Application Laid-Open (kokai) No. H11-43368), the porosity of the insulator is reduced to 0.5% or less.